Description:
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error_log is only used when daemonize=1.
This make sense to display message during an interactive / debug run.
Launching php-fpm using systemd (with type=simple of new type=notify), --nodaemonize option is used. So error_log directive is ignored and all messages goes to stdout (catched by systemd and redirected to system log).
Proposal : use error_log configured file if daemonize OR !interactive (using isatty test)